iter mq.2: type-driven dispatch mechanism installed (mechanism-before-exercise)

Installs the dispatch infrastructure for type-driven method
resolution without retiring MethodNameCollision yet. The new
multi-candidate path is exercised exclusively by 15 unit tests;
real workspaces continue producing single-class residuals
(candidates: None) and every pre-mq.2 fixture typechecks unchanged.

Three new CheckError variants:
- AmbiguousMethodResolution (multi-candidate after type-driven filter)
- UnknownClass (explicit qualifier names a class not in registry)
- NoInstance gains additive candidate_classes field (Vec<String>)

Schema/Env additions:
- Env.method_to_candidate_classes: BTreeMap<String, BTreeSet<String>>
  workspace-flat inverse of class_methods, built in build_check_env.
- ResidualConstraint.candidates: Option<BTreeSet<String>> — None
  preserves pre-mq.2 single-class semantics; Some(set) carries the
  multi-candidate residual that discharge refines.
- ResidualConstraint visibility bumped pub(crate) → pub for
  unit-test crate access.

New helpers:
- MethodDispatchOutcome enum + pure resolve_method_dispatch
  implementing the spec's 5-step rule (qualifier → singleton →
  type-driven filter → constraint-driven filter → Multi for
  discharge-time refinement).
- parse_method_qualifier splits Term::Var.name into
  (method_name, optional_qualifier_prefix) at the last dot.
- RefineOutcome enum + refine_multi_candidate_residual for
  discharge-time refinement.
- resolve_residual_class_for_mono wires the refinement into mono's
  collect_residuals_ordered residual-to-target mapping.

Synth Var-arm class-method branch rewritten via parse_method_qualifier
with inner-dot gate (qualifier must be <module>.<Class>; single-dot
names fall through to the existing qualified-fn path).

Constraint-discharge in check_fn uses expanded (post-superclass-
expansion) constraints for the rigid-var path — sounder than raw
declared_constraints.

Plan-invented format_type_for_display replaced with the existing
ailang_core::pretty::type_to_string (one less duplicate).

Synth-time declared_constraints: &[] is a deliberate gap documented
as known debt — load-bearing only post-mq.3 for the rigid-var
fallback (env-plumbing the active fn's constraints into the Var
arm is a ~10-line edit slated for mq.3).

9/9 tasks, 539 tests green (was 520 pre-mq.2; +15 mq.2 unit tests +
4 pre-existing). bench/compile_check.py + cross_lang.py clean;
bench/check.py 1 regression (latency noise — runtime cannot be
touched by a typecheck-side iter).

docs/journals/INDEX.md

@@ -43,3 +43,4 @@
 - 2026-05-12 — audit-ct-tidy: milestone close (ct-tidy) — architect drift fixed inline as `ctt.tidy` (3 doc-side edits: DESIGN.md §"Class-schema diagnostics" drops the retired `KindMismatch` bullet + adds successor paragraph naming `BareCrossModuleTypeRef`; DESIGN.md §"Higher-kinded class params" rewritten to name `BareCrossModuleTypeRef` from canonical-form validation instead; roadmap.md two P2 todos struck `[x]` with forward-reference to ctt.3 and ctt.2). Bench mixed (check.py exit 1: bump_s persistence on `bench_list_sum` second consecutive sighting at +12.23% → +12.60%; two new first-sighting rc-cohort max_us latency regressions classified as noise pending next audit; the recurring 3-audit `latency.explicit_at_rc` improvement cluster narrowed to within tolerance this audit, withdrawing the ratify-pending plan from audit-eob — the meaningful shrinkage is itself attribution evidence that the cluster is noise-class not signal-class), baseline pristine for the 4th consecutive audit → 2026-05-12-audit-ct-tidy.md
 - 2026-05-12 — iter 24.1: `bool_to_str` + `str_clone` runtime + codegen wiring — 2 new C functions in `runtime/str.c` (slab-allocating heap-Str primitives via existing `str_alloc`), lockstep checker + synth.rs installs with `ret_mode: Own`, 2 IR-header declares + 2 `lower_app` arms + `is_static_callee` whitelist extension, 5 IR snapshots regen (2 declares × 5 files), 9 new tests (2 builtins-install unit + 2 IR-shape pin + 5 E2E all green), pre-existing-drift fix included (int_to_str row added to `builtins.rs::list()`). One substantive deviation: builtin-signatures registered in `uniqueness.rs::infer_module` + `linearity.rs::check_module_with_visible` (8 LOC × 2 files) so `str_clone`'s `param_modes: [Borrow]` is visible to the App-arg walker; symmetric to 23.4-prep's class-method registration; necessary for the plan's literal `frees == 3` cross-realisation assertion. Full `cargo test --workspace` 513 passed, 0 failed. `bench/compile_check.py` + `bench/cross_lang.py` green → 2026-05-12-iter-24.1.md
 - 2026-05-13 — iter mq.1: class-ref canonical-form extension + workspace-internal class-name qualification — three schema fields (`InstanceDef.class`, `Constraint.class`, `SuperclassRef.class`) move bare → canonical (bare for same-module, `<module>.<Class>` for cross-module) symmetric to ct.1's `Type::Con.name` rule; `ClassDef.name` stays bare (defining site, like `TypeDef.name`). `validate_canonical_type_names` gains three field-walks via new `check_class_ref` helper plus two sibling diagnostics `BareCrossModuleClassRef` / `BadCrossModuleClassRef`; `check_class_name_fields` narrowed to `ClassDef.name`-only. Workspace-internal class-name keys all qualified: `class_def_module`, `class_by_name`, registry `entries.0`, `ClassMethodEntry.class_name`, `class_superclasses`, mono's `class_index`, `Origin::Class.class_name`. New `qualify_class_ref` helper in `workspace.rs` plus sibling `qualify_class_ref_in_check` in `ailang-check`. Two new positive on-disk fixtures (`mq1_xmod_constraint_class{,_dep}`). Recon claim that all existing fixtures' class-refs are intra-module was empirically wrong — 5 test_22b* (`orphan_third`, `dup_a/b/entry`, `unbound_constraint_var`) + 5 other (`eq_ord_polymorphic`, `eq_ord_user_adt`, `cmp_max_smoke`, `ctt2_collision_{lib,main}`) fixtures required minimal canonical-form migration. Duplicate-instance test architecturally restructured: post-mq.1 orphan-freedom makes two-modules-on-same-`(class, type)` structurally impossible (any second instance in a non-owning module fires `OrphanInstance` first); new `test_22b1_dup_same_module.ail.json` lands both instances in the class's module — the only post-mq.1 way to land two on the same canonical key. Plan defects fixed inline: `Origin::Class.class_name` had one construction site not two (plan recon off-by-one); `build_class_index` in `mono.rs` needed qualifying (plan said "no code change needed"); `build_module_globals` needed a `Def::Instance` carve-out for qualified `inst.class`; `check_fn`'s declared-constraint matching needed inline canonical-form lifting; `NoInstance` Float-aware message arm needed `prelude.Eq`/`prelude.Ord` match; coherence type-leg needed qualified-head split-and-lookup. Tasks 3-6 ran as one coherent fix (Task 3 alone left tests RED). 7/7 tasks, 520 tests green, `bench/compile_check.py` + `bench/cross_lang.py` exit 0; `bench/check.py` exit 1 due to 4 improvements-beyond-tolerance (audit-ratifiable per convention, not a regression). `MethodNameCollision` + dispatch path unchanged; iters mq.2 + mq.3 land them → 2026-05-13-iter-mq.1.md
+- 2026-05-13 — iter mq.2: type-driven dispatch mechanism installed (mechanism-before-exercise) — `Env.method_to_candidate_classes` workspace-flat inverse index built alongside `class_methods`; two new `CheckError` variants `AmbiguousMethodResolution` + `UnknownClass` (Display+code+ctx) plus additive `NoInstance.candidate_classes` field; `ResidualConstraint` extended with `candidates: Option<BTreeSet<String>>` (visibility bumped `pub(crate)` → `pub` for test-crate access); pure `resolve_method_dispatch` helper implementing the spec's 5-step rule (qualifier → singleton → type-driven filter → constraint-driven filter → `Multi` for discharge-time refinement); synth Var-arm class-method branch rewritten via `parse_method_qualifier` with inner-dot gate (qualifier must be `<module>.<Class>` form, single-dot names like `std_list.length` fall through to the existing qualified-fn path); `refine_multi_candidate_residual` wired into `check_fn` discharge using `expanded` (post-superclass-expansion constraints) for the rigid-var path; `resolve_residual_class_for_mono` wired into mono's `collect_residuals_ordered` residual-to-target mapping. With `MethodNameCollision` still gating real workspaces, the new multi-candidate branches are exercised exclusively by 15 unit tests: 6 in `tests/method_dispatch_pin.rs` covering the 5-step rule's six cases, 6 in `lib.rs` `mod tests` covering variants + field shapes + discharge refinement, 3 in `mono.rs` `mod tests` covering the mono helper. Real workspaces continue producing single-class residuals (`candidates: None`); every pre-mq.2 fixture typechecks unchanged. Plan-invented `format_type_for_display` replaced with `ailang_core::pretty::type_to_string` (one less duplicate). Synth-time `declared_constraints: &[]` is a deliberate gap documented as known debt — load-bearing only post-mq.3 for the rigid-var fallback (env-plumbing the active fn's constraints into the Var arm is a ~10-line edit slated for mq.3). 9/9 tasks, 539 tests green (was 520 at start of mq.2; +19 = 15 new + 4 pre-existing); `bench/compile_check.py` + `cross_lang.py` clean; `bench/check.py` 1 regression (latency noise, runtime cannot be touched by typecheck-side iter). `MethodNameCollision` retirement lands in mq.3 → 2026-05-13-iter-mq.2.md